Developer carrying member and image forming apparatus

ABSTRACT

Provided is a developer carrying member which depends on a sheet-passing speed to a small extent while maintaining satisfactory developability, and can stably suppress a fogging amount with time. The developer carrying member includes: an electro-conductive mandrel; and an electro-conductive elastic layer, in which: the elastic layer contains a resin j, an electro-semiconductive particle p, and an electro-conductive particle c; and when an electroconductivity of the resin j is defined as σ j , a dielectric constant of the resin j is defined as ∈ j , an electroconductivity of the electro-semiconductive particle p is defined as σ p  and a dielectric constant of the electro-semiconductive particle p is defined as ∈ p , σ j , ∈ j , σ p , and ∈ p  satisfy relationships represented by the following formulae (1) and (2), σ j , ∈ j , σ p  and ∈ p  being calculated by an AC impedance method.
 
σ j &lt;σ p &lt;0.05 S/cm  (1)
 
∈ p &lt;∈ j   (2)

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2015/003283, filed Jun. 30, 2015, which claims the benefit ofJapanese Patent Application No. 2014-134823, filed Jun. 30, 2014.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a developer carrying member and animage forming apparatus using the member.

In an image forming apparatus such as a laser printer using anelectrophotographic process, a contact developing system involving usinga developing roller having an elastic layer has been proposed as adeveloping system involving using a conventional one-component toner.FIG. 2 is a schematic view for illustrating the construction of an imageforming apparatus adopting the contact developing system involving usingthe developing roller having the elastic layer. Development is performedby causing a developing roller 14 as an elastic roller to carry anonmagnetic developer and bringing the developing roller into contactwith the surface of a photosensitive drum 1. The developer is suppliedto the developing roller by a supplying roller 15 in contact with thedeveloping roller. The supplying roller conveys the developer from theinside of a developer container 13 to adhere the developer to thedeveloping roller. In addition, the supplying roller serves to removethe developer remaining on the developing roller once. The layerthickness regulation of the developer adhering onto the developingroller and the provision of the developer with charge by triboelectriccharging are performed by bringing a toner-regulating member 16 intocontact with the developing roller. It has been proposed that thefollowing member of a blade shape be used as the toner-regulatingmember. The member supports a metal thin plate on one side and theventral surface of a portion opposite to the thin plate is brought intocontact with the developing roller. The developer with which thedeveloping roller has been coated by the toner-regulating memberdevelops an electrostatic latent image formed on the photosensitive drumwith the electric potential of a bias applied onto the developingroller.

In Japanese Examined Patent No. H7-31454, there is a disclosure thatfogging worsens owing to the loss of the charge of a toner particle, orthe inversion of its polarity, in a region where a photosensitive memberand a developing roller contact with each other.

SUMMARY OF THE INVENTION

It has been known that charge-providing performance for a toner reducesunder a high-humidity environment. Particularly in a one-componentcontact developing system involving providing the toner with chargethrough triboelectric charging with a toner-regulating member, aninfluence of the reduction in charge-providing performance for the toneris large because the toner has an extremely small number ofopportunities to obtain charge. As a result, a problem resulting fromthe reduction in charge-providing performance for the toner, e.g., anincrease in amount of fogging occurs. The “fogging” refers to such animage failure that the toner is slightly developed in a white portion(non-printing portion) where no character is originally printed toappear like smearing, and is a problem resulting from the reduction incharge-providing performance for the toner. In addition, in a regionwhere a photosensitive member and a developing roller contact with eachother through the toner, such a voltage that a force from thephotosensitive drum toward the developing roller acts on the tonerprovided with charge is applied in the non-printing portion. In JapaneseExamined Patent No. H7-31454, it is assumed that in the voltageapplication region where the photosensitive member and the developingroller contact with each other, the charge of the toner escapes towardthe developing roller owing to the voltage applied as described above,and hence the amount of fogging resulting from the attenuation of thetoner charge increases. An increase in volume resistance value of thedeveloping roller has been proposed as a method of suppressing thefogging amount. However, merely increasing the volume resistance valuecauses the deterioration of developability such as a low density.

The present invention has been made in view of the above-mentionedproblems, and the present invention is directed to providing a developercarrying member and an image forming apparatus each of which depends ona sheet-passing speed to a small extent while maintaining satisfactorydevelopability, and can stably suppress a fogging amount with time.

According to one aspect of the present invention, there is provided adeveloper carrying member, including: an electro-conductive mandrel; andan electro-conductive elastic layer, in which: the elastic layercontains a resin j, an electro-semiconductive particle p, and anelectro-conductive particle c; and when an electroconductivity of theresin j is defined as σ_(j), a dielectric constant of the resin j isdefined as ∈_(j), an electroconductivity of the electro-semiconductiveparticle p is defined as σ_(p) and a dielectric constant of theelectro-semiconductive particle p is defined as ∈_(p), σ_(j), ∈_(j),σ_(p), and ∈_(p) satisfy relationships represented by the followingformulae (1) and (2), σ_(j), ∈_(j), σ_(p) and ∈_(p) being calculated byan AC impedance method.σ_(j)<σ_(p)<0.05 S/cm  (1)∈_(p)<∈_(j)  (2)

Further, according to another aspect of the present invention, there isprovided an image forming apparatus, including: an image-bearing memberconfigured to bear an electrostatic latent image; and a developercarrying member configured to carry a developer and to contact with theimage-bearing member to develop the electrostatic latent image with thedeveloper, in which the developer carrying member is the above-mentioneddeveloper carrying member.

According to the aspects of the present invention, the developercarrying member and the image forming apparatus each of which depends ona sheet-passing speed to a small extent while maintaining satisfactorydevelopability, and can stably suppress a fogging amount with time canbe provided.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an image forming apparatus according tothe present invention.

FIG. 2 is a schematic view of a process cartridge according toEmbodiment 1.

FIG. 3 is a schematic view of the process cartridge according toEmbodiment 2.

FIG. 4 is a schematic view of a developer carrying member (developingroller) according to the present invention.

FIG. 5 is a view for illustrating the measurement of the AC impedance ofthe roller.

FIG. 6 is a view for illustrating the equivalent circuit model of thedeveloping roller.

FIG. 7 is a schematic sectional view of a developing roller model whoseelastic layer is formed of a three-layer structure.

FIG. 8 is a schematic view of the developing roller whoseelectro-conductive elastic layer contains a resin, anelectro-semiconductive particle, and an electro-conductive particle.

FIG. 9 is a schematic sectional view of the developing roller forillustrating methods of calculating an electroconductivity and adielectric constant.

FIG. 10 is a graph for showing the amount of a change in toner chargeafter passing through a portion where the developing roller and aphotosensitive drum contact with each other as compared with tonercharge before the passing.

FIG. 11 is a graph of the frequency characteristics of capacitances inExamples 1 to 3 and Comparative Example 1.

FIG. 12 is a schematic view for illustrating an electro-conductive pathin Example 1.

FIG. 13 is a schematic view for illustrating the electro-conductive pathwhen σ_(j)>σ_(p).

FIG. 14 is a schematic view for illustrating the electro-conductive pathwhen σ_(p)>0.05 S/cm.

FIG. 15 is a schematic view for illustrating the attenuation of thetoner charge between the developing roller and the photosensitive drum.

FIG. 16 is a schematic view for illustrating the synthesis of acomponent except an electro-semiconductive particle p component.

FIG. 17A is a schematic view for illustrating a difference in chargingprocess of charge.

FIG. 17B is a schematic view for illustrating a difference in chargingprocess of charge.

FIG. 17C is a schematic view for illustrating a difference in chargingprocess of charge.

FIG. 17D is a schematic view for illustrating a difference in chargingprocess of charge.

FIG. 18 is a graph for showing the results of the measurement of waterabsorption amounts in Examples 1 and 6.

FIG. 19 is a graph for showing temperature and humidity conditions inthe measurement of a water absorption amount.

FIG. 20A is a view for illustrating a method of producing a sample pieceof the developing roller used in the measurement of a water absorptionamount.

FIG. 20B is a view for illustrating the method of producing the samplepiece of the developing roller used in the measurement of a waterabsorption amount.

DESCRIPTION OF THE EMBODIMENTS

Now, a developer carrying member and an image forming apparatus to whichthe present invention is applied are described in detail.

[Image Forming Apparatus]

FIG. 1 is a schematic construction view of an image forming apparatusaccording to the present invention. The image forming apparatus is afull-color laser printer utilizing an electrophotographic process. Now,the entire schematic construction of the image forming apparatusaccording to the present invention is described by way of Embodiment 1and Embodiment 2; provided that the dimensions, materials, and shapes ofconstituent parts described in embodiments to be described below, therelative arrangement of the parts, and the like are not meant to limitthe scope of the present invention thereto unless otherwise stated.

Embodiment 1

An image forming apparatus applied to Embodiment 1 of the presentinvention is illustrated in FIG. 1. In addition, a cartridge 11constituting the image forming apparatus is illustrated in FIG. 2. Inthe image forming apparatus, the photosensitive member 1 as animage-bearing member is rotated in a direction indicated by an arrow,and is charged to a uniform electric potential Vd by a charging roller 2as a charging apparatus. Next, the photosensitive member is exposed bylaser light from a laser irradiation apparatus 3 as an exposingapparatus, whereby an electrostatic latent image is formed on itssurface. The electrostatic latent image is developed by a developingapparatus 4 to be visualized as a toner image. The visualized tonerimage on the photosensitive member is transferred onto an intermediatetransfer member 6 by a primary transfer apparatus 5, and is thentransferred onto paper 8 as a recording medium by a secondary transferapparatus 7. Transfer residual toner remaining on the photosensitivemember without being transferred is scraped off by a cleaning blade 9 asa cleaning apparatus. The cleaned photosensitive member repeats theabove-mentioned actions to perform image formation. Meanwhile, the paperonto which the toner image has been transferred is subjected to fixationby a fixing apparatus 10, and is then discharged to the outside of theimage forming apparatus.

As illustrated in FIG. 2, the photosensitive member 1, the chargingroller 2, the developing apparatus 4, and the cleaning blade 9 areintegrally constituted as the cartridge 11 detachably mountable to themain body of the image forming apparatus. Four portions on each of whichthe cartridge 11 is mounted are prepared for the main body of the imageforming apparatus. In addition, cartridges filled with yellow, magenta,cyan, and black toners are mounted from the upstream side of thedirection in which the intermediate transfer member 6 moves,respectively, and a color image can be formed by sequentiallytransferring the toners onto the intermediate transfer member.

[Image-Bearing Member]

The image-bearing member for bearing the electrostatic latent image is,for example, a photosensitive drum, and can be formed by a knownprocess. The photosensitive drum is of a construction in which thelayers of organic photosensitive members obtained by applying a positivecharge injection-preventing layer, a charge-generating layer, and acharge-transporting layer in the stated order in a superimposed mannerare laminated on a cylinder as an electro-conductive substrate. Forexample, a polyarylate is used as the charge-transporting layer, and thethickness of the charge-transporting layer is adjusted to about 23 μm.The charge-transporting layer is formed by dissolving acharge-transporting material in a solvent together with a binder. As anorganic charge-transporting material, there may be given, for example,an acrylic resin, a styrene-based resin, polyester, a polycarbonateresin, polyarylate, polysulfone, polyphenylene oxide, an epoxy resin, apolyurethane resin, an alkyd resin, and an unsaturated resin. One kindof these charge-transporting materials may be used alone, or two or morekinds thereof may be used in combination.

[Charging Apparatus]

The charging roller constituting the charging apparatus is of, forexample, a construction in which an electro-semiconductive rubber layeris arranged on a cored bar as an electro-conductive support, and theelectric resistance value of the charging roller is set to, for example,about 10⁵Ω when a voltage of 200 V is applied to the electro-conductivephotosensitive drum.

[Developing Apparatus]

The developing apparatus 4 includes a toner 12 as a developer, thedeveloper container 13 as a developer-storing portion, the developingroller 14 as a developer carrying member, the supplying roller 15 forsupplying the toner to the developing roller, and the regulating blade16 as a developer-regulating member for regulating the toner on thedeveloping roller. Details about the developing roller are describedlater. The supplying roller rotates while contacting with the developingroller, and one end of the regulating blade 16 contacts with thedeveloping roller.

The supplying roller 15 is of a construction in which a foamed urethanelayer 15 b is arranged on the periphery of a cored bar electrode 15 a asan electro-conductive mandrel. The outer diameter of the cored barelectrode is, for example, 5.5 mm. The outer diameter of the entirety ofthe supplying roller including the foamed urethane layer is, forexample, 13 mm. The amount in which the supplying roller and thedeveloping roller penetrate each other is 1.2 mm. The supplying rollerrotates in such a direction that the roller and the developing rollerhave velocities in directions opposite to each other in a portion wherethe rollers contact with each other. The powder pressure of the toner 12present around the foamed urethane layer acts on the layer and thesupplying roller rotates, whereby the toner is taken in the foamedurethane layer. The supplying roller containing the toner supplies thetoner to the developing roller in the contacting portion with thedeveloping roller, and rubs the toner to provide the toner withpreliminary triboelectric charge. Meanwhile, the supplying roller forsupplying the toner to the developing roller has a role of peeling thetoner remaining on the developing roller without being developed in adeveloping portion.

The toner supplied from the supplying roller to the developing rollerreaches the regulating blade, and its charge quantity and layerthickness are adjusted to desired ones. The regulating blade is, forexample, a SUS blade having a thickness of 80 μm, and is placed in adirection against the rotation of the developing roller. The amount ofthe toner on the developing roller is regulated by the regulating bladeand hence a uniform toner layer thickness is obtained. In addition, adesired charge quantity is obtained by triboelectric charging based onrubbing. In addition, a voltage having a potential difference of, forexample, −200 V with respect to the developing roller is applied to theregulating blade. The potential difference is intended for thestabilization of a toner coat layer.

A toner layer formed on the developing roller by the regulating blade isconveyed to the developing portion contacting with the photosensitivedrum, and reversal development is performed in the developing portion.At the contacting position, the amount in which the developing rollerpenetrates the photosensitive drum is set to, for example, 40 μm by aroller (not shown) in an end portion of the developing roller. Thesurface of the developing roller is pressed against the photosensitivedrum to deform, thereby forming a developing nip. Thus, development canbe performed in a stable contacting state. In the developing nip, thedeveloping roller rotates in the same direction as that of thephotosensitive drum at a peripheral speed ratio of 117% with respect tothe photosensitive drum. It is because such peripheral speed differencehas a role of stabilizing the amount of the toner to be developed thatthe difference is made.

[Developer]

The developer that can be used in the image forming apparatus of thepresent invention, which is not particularly limited, is, for example, aone-component nonmagnetic toner. The one-component nonmagnetic toner isprepared so as to contain a binder resin and a charge control agent, andis produced so as to have negative polarity through the addition of afluidizer or the like as an external additive. The toner is produced bya polymerization method and its average particle diameter is adjustedto, for example, about 5 μm.

[Developer Carrying Member]

A developer carrying member according to the present invention has arole of contacting with an image-bearing member bearing an electrostaticlatent image to develop the electrostatic latent image with a developer.Now, the developer carrying member of the present invention is describedin detail by way of a developing roller as a typical form of thedeveloper carrying member. As illustrated in FIG. 4, the developingroller has at least an electro-conductive mandrel and anelectro-conductive elastic layer. In addition, the roller can have asurface layer as required. An example of a sectional view of thedeveloping roller is illustrated in FIG. 8.

(Electro-Conductive Mandrel)

A material for an electro-conductive mandrel 14 a is not particularlylimited as long as the material is electro-conductive, and a materialappropriately selected from carbon steel, alloy steel, cast iron, and anelectro-conductive resin can be used. Examples of the alloy steelinclude stainless steel, nickel-chromium steel,nickel-chromium-molybdenum steel, chromium steel, chromium-molybdenumsteel, and nitriding steel having added thereto Al, Cr, Mo, and V.

(Electro-Conductive Elastic Layer)

An electro-conductive elastic layer 14 b contains at least a resin j, anelectro-semiconductive particle p, and an electro-conductive particle c.The electro-conductive elastic layer is arranged for imparting, to thedeveloping roller, elasticity required in an apparatus to be used. Theconstruction of the layer may be specifically, for example, any one of asolid body and a foam. In addition, the elastic layer may be a singlelayer, or may be formed of a plurality of layers. For example, thedeveloper carrying member is always in press contact with thephotosensitive member and the toner, and hence an elastic layer havingthe following characteristics is arranged for alleviating damagemutually done between these members: low hardness and a low compressionset.

[Resin j]

Examples of the resin j include one kind selected from, for example,urethane rubber, chloroprene rubber, isoprene rubber, butadieneacrylonitrile, epichlorohydrin rubber, ethylene propylene rubber, hydrinrubber, fluororubber, natural rubber, butyl rubber, nitrile rubber,polyisoprene rubber, polybutadiene rubber, silicone rubber,styrene-butadiene rubber, ethylene-propylene rubber, chloroprene rubber,and acrylic rubber, and mixtures of two or more kinds thereof. Inparticular, urethane rubber, chloroprene rubber, butadieneacrylonitrile, epichlorohydrin rubber, or the like is preferred. Onekind of these resins may be used alone, or two or more kinds thereof maybe used in combination.

[Electro-Semiconductive Particle p]

A material for the electro-semiconductive particle p is, for example,any one of the following materials: metal oxides such as silica, zincoxide, titanium oxide, aluminum oxide, tin oxide, antimony oxide, indiumoxide, and silver oxide. The electroconductivity of theelectro-semiconductive particle p is preferably from 1×10⁻¹¹ S/cm to1×10⁻³ S/cm.

[Electro-Conductive Particle c]

As a material for the electro-conductive particle c, there are given,for example: electro-conductive carbons such as carbon black andacetylene black; carbons for rubber such as SAF, ISAF, HAF, FEF, GPF,SRF, FT, and MT; a carbon for a color subjected to oxidization treatmentor the like, a pyrolytic carbon; indium-doped tin oxide (ITO); metalssuch as copper, silver, and germanium; and electro-conductive polymerssuch as polyaniline, polypyrrole, and polyacetylene. Theelectroconductivity of the electro-conductive particle c is preferablyfrom 1×10⁻² S/cm to 1×10³ S/cm. In addition, the content of theelectro-conductive particle c in the electro-conductive elastic layer ispreferably from 5 mass % to 30 mass %.

In addition, the electro-conductive elastic layer may contain anelectroconductivity-imparting agent. Examples of theelectroconductivity-imparting agent include the following ion conductivesubstances: inorganic ion conductive substances such as sodiumperchlorate, lithium perchlorate, calcium perchlorate, and lithiumchloride; and organic ion conductive substances such as modified fattyacid dimethylammonium ethosulfate, stearic acid ammonium acetate,laurylammonium acetate, and octadecyltrimethylammonium perchlorate salt.

The resin j, the electro-semiconductive particle p, and theelectro-conductive particle c incorporated into the electro-conductiveelastic layer are preferably a urethane resin, a zinc oxide particle,and a carbon particle, respectively. The electro-conductive elasticlayer formed of such material combination has the following advantages:an effect in the present invention is stably obtained and the layer canbe produced at a low cost.

In addition, the volume occupancy of the zinc oxide particle in theelectro-conductive elastic layer of the above-mentioned constructionpreferably increases toward the vicinity of the surface of theelectro-conductive elastic layer. When the zinc oxide particle is placedin the electro-conductive elastic layer as described above, a reductionin capacitance of the developer carrying member not only exhibits asuppressing effect on the attenuation of the charge of a toner but alsoimproves the charge-providing performance for the toner by the zincoxide particle. As a result, fogging can be effectively andsignificantly suppressed.

[Surface Roughness]

As a guideline, the surface roughness of the developing roller ispreferably as follows: a center line average roughness Ra in thestandard of JIS B 0601:1994 “Surface Roughness” is from 0.3 μm to 5.0μm. Setting the Ra to 0.3 μm or more can provide an additionally stabledeveloper coating amount and hence contributes to the formation of ahigh-quality electrophotographic image having a uniform image density.

[Electroconductivity and Dielectric Constant]

In the electro-conductive elastic layer constituting the developercarrying member of the present invention, electroconductivities σ_(j)and σ_(p), and dielectric constants ∈_(j) and ∈_(p) of the resin j andthe electro-semiconductive particle p calculated by an AC impedancemethod have relationships represented by the following formulae (1) and(2).σ_(j)<σ_(p)<0.05 S/cm  (1)∈_(p)<∈_(j)  (2)

Satisfying the relationship of σ_(j)<σ_(p) represented by the formula(1) results in the formation of an electro-conductive path passing theelectro-semiconductive particle p, and satisfying the relationship ofσ_(p)<0.05 S/cm can cause the dielectric characteristics of theelectro-semiconductive particle p to act on the electro-conductive path.In addition, the relationship represented by the formula (2) issatisfied and hence the following effect can be obtained: the dielectriccharacteristics of the electro-semiconductive particle p reduce acapacitance C of the developer carrying member. As a result, theattenuation of the charge of the toner and fogging can be suppressed.

It should be noted that in the formula (1), [S/cm] is the unit of anelectroconductivity meaning [siemens per centimeter] and “S” isidentical in meaning to “1/Ω”.

The thickness of the electro-conductive elastic layer in the presentinvention is preferably set to be larger than the particle diameter ofthe electro-semiconductive particle p. Setting the thickness to a valuelarger than the particle diameter of the electro-semiconductive particlep can effectively suppress the attenuation of the toner charge. Inaddition, the thickness of the electro-conductive elastic layer issmaller than preferably 0.01 times, particularly preferably 0.003 timesthe outer diameter of the developer carrying member. Setting thethickness of the electro-conductive elastic layer to a value smallerthan 0.01 times the outer diameter of the developer carrying membersuppresses an influence of the expansion of the member in its radialdirection and hence can enjoy the suppressing effect on the attenuationof the toner charge with additional reliability.

When the volume fractions of the electro-conductive particle c, theelectro-semiconductive particle p, and the resin j with respect to theentirety of the electro-conductive elastic layer are defined as Vc, Vp,and Vj, respectively, the ratio Vp/Vc of the Vp to the Vc is preferablylarger than 0.5. Setting the ratio Vp/Vc to a value larger than 0.5suppresses a reduction in ratio of the electro-semiconductive particle ppassing the electro-conductive path and hence can reduce the capacitanceof the developer carrying member. In addition, the ratio Vp/Vj of the Vpto the Vj is preferably larger than 0.3 and smaller than 0.8. Settingthe ratio Vp/Vj to a value larger than 0.3 suppresses a reduction inratio of the electro-semiconductive particle p to the resin j and hencecan reduce the capacitance of the developer carrying member. Inaddition, setting the ratio Vp/Vj to a value smaller than 0.8 suppressesthe agglomeration of the electro-semiconductive particles p andfacilitates their uniform dispersion in the resin j, and hence caneffectively reduce the capacitance of the developer carrying member. Inorder that the reducing effect on the capacitance of the developercarrying member in the present invention may be obtained, the ratioVp/Vj is more preferably larger than 0.4 and smaller than 0.7.

The resistance value of the developer carrying member is desirably from2×10⁴Ω to 5×10⁷Ω. When the resistance value is 2×10⁴Ω or more, thephenomenon in which the quantity of a current flowing through theelastic layer increases and hence a required current quantity becomesexcessively large can be suppressed. In addition, when the resistancevalue is set to 5×10⁷Ω or less, a current flowing at the time ofdevelopment is hardly inhibited. The resistance value of the developercarrying member to be used in the present invention is set to, forexample, 5×10⁵Ω through the adjustment of the addition amount of theelectro-conductive particle.

The resistance value of the developer carrying member is calculated fromthe result of the measurement of a complex impedance characteristic.Analysis is performed with an equivalent circuit model illustrated inFIG. 6 in which parallel equivalent circuits each having a conductanceand a capacitance are connected in series, the value at which an angularfrequency ω of a real part Z′ of the complex impedance characteristicbecomes 0 is derived, and the derived value is defined as the resistancevalue of the developer carrying member. After the developer carryingmember has been left to stand in an evaluation environment (having atemperature of 30° C. and a relative humidity of 80%) for 12 hours, themeasurement of the complex impedance characteristic is performed underthe environment. For example, “K. S. Zhao, K. Asaka, K. Asami, T. Hanai,Bull. Inst. Chem. Res., Kyoto Univ., 67 225-255 (1989)” is given as areference relating to the measurement of the complex impedancecharacteristic.

<Methods of Measuring Conductance and Capacitance Characteristics>

Methods of measuring physical property values to be used in the presentinvention are described below.

A conductance G, the capacitance C, an electroconductivity σ, and adielectric constant ∈ are measured with an AC impedance analyzer(manufactured by Solartron, 1260 type impedance analyzer+1296 typedielectric constant-measuring interface). An AC voltage of 500 mV issuperimposed on a DC voltage of 20 V, and the complex impedancecharacteristic is measured with respect to a change in frequency from 1MHz to 1 Hz. The frequency characteristics of the conductance G and thecapacitance C are determined based on relational formulae (3) and (4)concerning the real part Z′ and imaginary part Z″ of the compleximpedance characteristic, and the conductance G, the capacitance C, andthe angular frequency ω.

$\begin{matrix}{G = \frac{Z^{\prime}}{Z^{\prime\; 2} + Z^{''\; 2}}} & (3) \\{C = {\frac{1}{\omega}\frac{Z^{''}}{Z^{\prime\; 2} + Z^{''\; 2}}}} & (4)\end{matrix}$

The outline of the developer carrying member in the measurement of thecomplex impedance characteristic is illustrated in FIG. 5. Threeelectro-conductive tapes each having a width of 15 mm are wound aroundthe surface of the developer carrying member at an interval of 1 mm asillustrated in FIG. 5. An electro-conductive tape D2 positioned at acenter out of the three electro-conductive tapes and the mandrel of thedeveloper carrying member are defined as a main electrode, two outsideelectro-conductive tapes D1 and D3 are defined as guard electrodes, andthe electrodes are used in the measurement. After the developer carryingmember has been left to stand in an evaluation environment (having atemperature of 30° C. and a relative humidity of 80%) for 12 hours, themeasurement of the complex impedance characteristic is performed underthe environment.

<Equivalent Circuit Model of Complex Impedance Characteristic>

Conductances G₁ to G_(n) and capacitances C₁ to C_(n) are derived fromthe complex impedance characteristic through analysis with theequivalent circuit model illustrated in FIG. 6 in which the parallelequivalent circuits of the conductances and the capacitances areconnected in series. The analysis is performed with an impedanceanalysis software Zview (manufactured by Solartron). The inventors ofthe present invention have made extensive investigations, and as aresult, have found that the complex impedance characteristic of adeveloping roller formed of such constituent materials as illustrated inFIG. 8 can be generally approximated to the complex impedancecharacteristic of such a developing roller as illustrated in FIG. 7 inwhich the layer of a material 1 (base layer), the layer of a material 2(layer of the electro-semiconductive particle p), and the layer of amaterial 3 (layer of the resin j) are formed around a mandrel. Apossible reason for the foregoing is described below based on adeveloping roller used in Example 1 to be described later. It should benoted that in Example 1, an electro-conductive elastic layer isconstituted of a surface layer containing the resin j, theelectro-semiconductive particle p, and the electro-conductive particlec, and a base layer.

Simply thinking, the electro-conductive path of a current when amaterial constituting the electro-conductive elastic layer of adeveloping roller is a system containing mainly carbon, urethane as aresin, and an electro-semiconductive particle like Example 1 is formedby the summation of, for example, electro-conductive paths betweencarbon and the resin, between carbon and the electro-semiconductiveparticle, and between the resin and the electro-semiconductive particlelike the section (a) of FIG. 12. Meanwhile, a capacitance C_(tot) of theentirety of the roller is represented by C_(tot) (C₁, . . . C_(n), G₁, .. . G_(n)). However, when the respective electro-conductive paths areincluded in a surface layer having a thickness of about 10 μm, aninfluence of the expansion of the roller in its radial direction can beapproximated to a plane, and is substantially equivalent to a suffix.Accordingly, as illustrated in the section (b) of FIG. 12, the C_(tot)does not change even when the order of layers corresponding to therespective electro-conductive paths is changed. Accordingly, when pathsof the same kind are mixed in such a series of electro-conductive pathsas illustrated in the section (b) of FIG. 12, the electro-conductivepaths of the same kind can be integrated into one path as illustrated inthe section (c) of FIG. 12. In the developing roller to be used inExample 1, its surface layer alone can be attributed to a two-layermodel (urethane and the electro-semiconductive particle p), and layersincluding the surface layer and a silicone resin layer as the base layercan be attributed to a three-layer model (the silicone resin layer isadded). Here, carbon has a low resistance and can be regarded as aconductor, and hence the extent to which carbon affects the analysis ofthe electrical characteristics of the entirety of the roller is assumedto be small. Therefore, the complex impedance characteristic of adeveloping roller like this example can be analyzed with suchthree-layer model as illustrated in the section (c) of FIG. 12.Specifically, the analysis can be performed with a (three-phase)equivalent circuit model in which three parallel equivalent circuitseach having a conductance and a capacitance are connected in series. Inthis example, when the error of the calculation of the layer formed ofthe electro-semiconductive particle p was 10% or less, the layer wasregarded as being attributable to such model.

Next, methods of deriving an electroconductivity σ_(i) and a dielectricconstant ∈_(i) corresponding to the conductance G_(i) and thecapacitance C_(i) thus derived, respectively are described.

<Methods of Deriving Electroconductivity σ and Dielectric Constant ∈>

The electroconductivity σ and the dielectric constant ∈ are calculatedfrom the conductance G=σS/d and the capacitance C=∈S/d derived from theanalysis of the complex impedance characteristic by using the followingrelational formulae (5) and (6).

Parameters a, b, and x of each of the relational formulae (5) and (6)are described below by taking FIG. 7 as an example. In order that theelectroconductivity σ of the resin j illustrated in FIG. 7 may becalculated, the conductance G derived by the above-mentioned analysismethod, a distance from the center of the mandrel to the surface of thebase layer as the parameter a, a distance from the center of the mandrelto the surface of the layer of the resin j as the parameter b, and thewidth of the electro-conductive tape D2 illustrated in FIG. 5 (=1.5×10⁻²[m]) as the parameter x are substituted into the relational formula (5).When the dielectric constant ∈ is calculated, the parameters a, b, and xare similarly substituted. Here, ∈₀ represents a dielectric constant ofvacuum and is 8.854×10⁻¹² [F/m]. The relational formulae (5) and (6)show that the conductance G and capacitance C of the developer carryingmember to be determined are represented by the series connection of aconductance ΔG and a capacitance ΔC of a cylindrical shape having adistance r [m] from the mandrel, a minute thickness dr [m], and anelectrode area 2πr·x [m²] illustrated in FIG. 9.

$\begin{matrix}\begin{matrix}{{\frac{1}{G} = {\int_{a}^{b}\frac{1}{\Delta\; G}}}\ } & {= {\frac{1}{2\pi\;{x \cdot \sigma}}\ln\frac{b}{a}}} \\{{= {\int_{a}^{b}\frac{1}{\sigma \cdot \frac{2\pi\;{r \cdot x}}{\mathbb{d}r}}}}\ } & {{\therefore\sigma} = \frac{{G \cdot \ln}\frac{b}{a}}{2\pi\; x}} \\{= {\frac{1}{2\pi\;{x \cdot \sigma}}\left\lbrack {\ln\mspace{11mu} r} \right\rbrack}_{a}^{b}} & \;\end{matrix} & (5) \\\begin{matrix}{{\frac{1}{C} = {\int_{a}^{b}\frac{1}{\Delta\; C}}}\ } & {= {\frac{1}{2\pi\;{x \cdot ɛ_{0}}ɛ_{r}}\ln\frac{b}{a}}} \\{{= {\int_{a}^{b}\frac{1}{ɛ_{0}ɛ_{r}\frac{2\pi\;{r \cdot x}}{\mathbb{d}r}}}}\ } & {{\therefore ɛ_{r}} = \frac{{C \cdot \ln}\frac{b}{a}}{2\pi\;{x \cdot ɛ_{0}}}} \\{= {\frac{1}{2\pi\;{x \cdot ɛ_{0}}ɛ_{r}}\left\lbrack {\ln\mspace{11mu} r} \right\rbrack}_{a}^{b}} & \;\end{matrix} & (6)\end{matrix}$

Next, methods of calculating the electroconductivity σ and dielectricconstant ∈ of the electro-semiconductive particle p in the developingroller illustrated in FIG. 8 formed of the resin j, and theelectro-semiconductive particle p and the electro-conductive particle cincorporated into the resin are described. In the developing roller ofthe construction illustrated in FIG. 8, values estimated from the volumefractions of the electro-semiconductive particle and theelectro-conductive particle with respect to the resin are substitutedinto the parameters a and b of each of the relational formulae (5) and(6). In other words, the construction of FIG. 8 is also approximated tothe construction of FIG. 7, and the parameters a and b are calculatedfrom the volume fractions. The volume fractions used here are the volumeratios of the respective materials (the resin j, theelectro-semiconductive particle p, and the electro-conductive particlec) with respect to the entirety of the surface layer obtained by theobservation of a section of the developing roller with a transmissionelectron microscope and the identification of the respective materials.

Here, aj and ap of the resin j and the electro-semiconductive particle peach corresponding to the parameter a, and bj and by of the resin andthe particle each corresponding to the parameter b are defined asfollows: ap=5.65×10⁻³ [m] (distance from the center of the mandrel tothe surface of the base layer), bp=ap+surface layer thickness×(volumefraction of the electro-semiconductive particle), aj=bp, andbj=aj+surface layer thickness×(volume fraction of the resin j).

<Method of Deriving σ_(u)/∈_(u)>

In the electro-conductive elastic layer of the present invention,electroconductivities σ_(p) and σ_(u), and dielectric constants ∈_(p)and ∈_(p) of the electro-semiconductive particle p component and acomponent u except the electro-semiconductive particle p, theelectroconductivities and the dielectric constants being calculatedthrough the separation of the constituent components of the layer intothe electro-semiconductive particle p component and the component uexcept the electro-semiconductive particle p by the AC impedance method,preferably have a relationship represented by the following formula(10). When the relationship represented by the formula (10) issatisfied, the frequency dependence of the capacitance C of thedeveloper carrying member reduces. As a result, the following effect canbe obtained: even when the process speed of an image forming apparatusat the time of its image formation is changed, a fogging amount isstably suppressed.|log₁₀[(σ_(p)/∈_(p))/(σ_(u)/∈_(u))]|<1.5  (10)

In a developing roller having a complex impedance characteristicdescribed by such three-phase equivalent circuit as illustrated in thesection (c) of FIG. 12, when a synthetic layer u of the layers exceptthe electro-semiconductive particle p (resin j+base layer) isconsidered, the roller can be considered as a two-phase model formed ofthe electro-semiconductive particle p and the synthetic layer u (FIG.16). By assuming that the ratio of a conductance G_(u) of the syntheticlayer u to its capacitance C_(u) is equal to the ratio of theelectroconductivity σ_(u) of the synthetic layer to its dielectricconstant ∈_(u), the ratio can be represented as follows:G_(u)/C_(u)=σ_(u)/∈_(u). The ratio σ_(u)/∈_(u) of the synthetic layer ucan be calculated by: substituting a conductance G and a capacitanceC_(j) of the resin j, and a conductance G_(b) and a capacitance C_(b) ofthe base layer into the following relational formulae (7) and (8) tocalculate the conductance G_(u) and capacitance C_(u) of the syntheticlayer u; and substituting the parameters into the relational formula(9). Here, ∈₀ represents a dielectric constant of vacuum and is8.854×10⁻¹² [F/m].

$\begin{matrix}{C_{u} = \frac{{C_{j}G_{b}^{2}} + {C_{b}G_{j}^{2}}}{\left( {G_{j} + G_{b}} \right)^{2}}} & (7) \\{G_{u} = \frac{G_{j}G_{b}}{G_{j} + G_{b}}} & (8) \\{\frac{\sigma_{u}}{ɛ_{u}} = {\frac{G_{u}}{C_{u}} \cdot ɛ_{0}}} & (9)\end{matrix}$

(Surface Layer)

When the developer carrying member of the present invention has asurface layer on the electro-conductive elastic layer, the same resin asthe resin j can be used as a material constituting the surface layer.

<Method of Producing Developer Carrying Member>

A method of producing a developer carrying member according to thepresent invention, which is not particularly limited, is, for example, amethod involving: dispersing and mixing materials for forming anelectro-conductive elastic layer in a solvent to prepare a coatingmaterial; applying the coating material onto an electro-conductivemandrel; and drying the resultant coating film to solidify the film orcuring the film. The following dispersion apparatus may be suitablyutilized for dispersing and mixing: a known medium dispersion apparatussuch as a ball mill, a sand mill, an attritor, or a bead mill; or aknown medium-less dispersion apparatus utilizing a collision-typeatomization method or a thin-film spinning method. In addition, thefollowing known method is applicable as an application method for theobtained coating material: a dipping method, a spraying method, a rollcoating method, an electrostatic application, or the like.

A method of forming a surface layer that can be formed on theelectro-conductive elastic layer as required, which is not particularlylimited, is, for example, a method involving: dispersing and mixing eachcomponent of the surface layer in a solvent to prepare a coatingmaterial; applying the coating material onto the electro-conductiveelastic layer; and drying the resultant coating film to solidify thefilm or curing the film. Any known dispersion apparatus utilizing beadssuch as a sand mill, a paint shaker, DYNO-MILL, or a pearl mill may besuitably utilized for dispersing and mixing each component. In addition,any known method such as a dipping method, a spraying method, or a rollcoating method may be applicable as an application method of theobtained coating material to the electro-conductive elastic layer.

<Image Forming Method>

Image formation by the image forming apparatus of Embodiment 1 can beperformed by, for example, the following method.

The amount of a toner to be filled into a developing apparatus is setto, for example, an amount corresponding to such an amount that aconverted image having an image ratio of 5% can be printed on 3,000sheets. The horizontal line having an image ratio of 5% is specifically,for example, such an image that the following pattern is repeated: aftera one-dot line has been printed, no lines are printed over 19 dots.

In an image forming process, the photosensitive drum is rotationallydriven at a speed of 120 mm/sec by the image forming apparatus in adirection indicated by an arrow r in FIG. 1. In addition, the imageforming apparatus has a low-speed mode corresponding to a process speedof 60 mm/sec in order that a quantity of heat for fixation may besecured at the time of the passing of thick recording paper (thickpaper). In addition, in this embodiment, the apparatus operatesaccording to only the two kinds of process modes, but may have aplurality of process modes depending on recording paper, and may beconstructed so as to be capable of performing control corresponding toeach of the process modes.

Next, a specific voltage in this embodiment is described. The surface ofthe photosensitive drum is uniformly charged to −500 V by applying avoltage of −1,050 V to the charging roller, whereby a dark potential(Vd) is formed. The electric potential of a printing portion is adjustedto −100 V (light potential V1) by laser as exposing means. At this time,a voltage (Vdc) of −300 V is applied to the developing roller totransfer the negative polarity toner into a light potential, wherebyreversal development is performed.

In addition, the value of |Vd−Vdc| is referred to as “Vback” and theVback is set to, for example, 200 V.

Embodiment 2

FIG. 3 is a schematic construction view for illustrating a processcartridge of a second embodiment of the present invention. An imagerecording apparatus of this embodiment is a laser printer of a tonerrecycle process (cleaner-less system) utilizing a transfer-typeelectrophotographic process. The redescription of the same points asthose of the image recording apparatus of Embodiment 1 is omitted, anddifferent points are described. A characteristic point in thisembodiment lies in that transfer residual toner is recycled without theplacement of any cleaning blade. The transfer residual toner is recycledso as not to adversely affect any other process such as charging,whereby the toner is recovered in a developing device. Specifically, thefollowing constructions are changed for Embodiment 1.

The same roller as that of Embodiment 1 is used as the charging roller 2constituting a charging apparatus, but the charging apparatus furtherincludes a contacting member 17 for the purpose of preventing thecharging roller from being contaminated with a toner. Even when thecharging roller is contaminated with a toner having polarity (positivepolarity) opposite to the charged polarity of the roller, the toner ischarged from positive charge to negative charge and the toner is quicklyejected from the charging roller, and hence the toner can be recoveredin the developing device by simultaneous development cleaning. Forexample, a polyimide film having a thickness of 100 μm is used as thecontacting member, and is brought into contact with the charging rollerat a linear pressure of 10 (N/m) or less. The polyimide has thetriboelectric charging characteristic by which the toner is providedwith negative charge. In addition, the absolute value of thenon-exposure potential Vd and the value of the Vback are set to be largein order that the property by which the toner is recovered in thedeveloping device may be improved. Specifically, the electric potentialof the surface of the photosensitive drum is set to the uniform electricpotential Vd=−800 V by setting the voltage to be applied to the chargingroller to −1,350 V. Further, the Vback is set to 500 V by setting adeveloping bias to −300 V.

Example 1

A developing roller of a structure illustrated in FIG. 4 was produced asdescribed below.

Used as the electro-conductive mandrel 14 a was a product obtained byplating a cored bar made of SUS22 having an outer diameter of 6 mm and alength of 26.5 mm with nickel, and applying and baking PRIMER DY35-051(trade name, manufactured by Dow Corning Toray Silicone Co., Ltd.) tothe resultant. The electro-conductive rubber layer 14 b blended with anelectro-conductive particle and an electro-semiconductive particle wasarranged on the periphery of the mandrel to set the outer diameter ofthe developing roller 14 to 11.5 mm. Materials for the rubber layer(electro-conductive elastic layer) were as follows: a first layer was asilicone rubber layer (thickness: 2.74 mm) and a second layer was aurethane layer (thickness: 10 μm). The urethane layer was formed of aZnO particle as the electro-semiconductive particle, a carbon blackparticle as the electro-conductive particle, and a urethane resin.

Details about a method of forming the silicone rubber layer, methods ofsynthesizing a polyol and an isocyanate as raw materials for theurethane resin, and a method of forming the urethane layer are asdescribed below.

[1. Formation of Silicone Rubber Layer]

The electro-conductive mandrel was placed in a cylindrical die having aninner diameter of 11.48 mm so as to be concentric therewith, and anaddition-type silicone rubber composition having formulation shown inTable 1 below was injected into a cavity formed in the die. The amountof a silica powder as a filler was adjusted for adjusting the hardnessof the entirety of the developing roller.

TABLE 1 Material Part(s) by mass Liquid silicone rubber materialSE6724A/B 100 (trade name, manufactured by Dow Corning Toray SiliconeCo., Ltd.) Carbon black TOKABLACK #7360SB (trade 35 name, manufacturedby Tokai Carbon Co., Ltd.) Silica powder 0.2 Platinum catalyst 0.1

Subsequently, the silicone rubber was vulcanized and cured at 150° C.for 15 minutes by heating the die, and was removed from the die. Afterthat, a curing reaction was completed by further heating the resultantat 200° C. for 2 hours. Thus, the silicone rubber layer having athickness of 2.74 mm was arranged on the outer periphery of theelectro-conductive mandrel.

[2. Synthesis of Polyol]

100 Parts by mass of a polytetramethylene glycol PTG1000SN (trade name,manufactured by Hodogaya Chemical Co., Ltd.) was mixed with 20 parts bymass of an isocyanate compound MILLIONATE MT (trade name, manufacturedby Nippon Polyurethane Industry Co., Ltd.) in a methyl ethyl ketone(MEK) solvent in a stepwise manner. The mixed solution was subjected toa reaction under a nitrogen atmosphere at 80° C. for 7 hours to producea polyether polyol having a hydroxyl value of 20 [mgKOH/g].

[3. Synthesis of Isocyanate]

Under a nitrogen atmosphere, 100 parts by mass of a polypropylene glycolhaving a number-average molecular weight of 400 (trade name: EXCENOL,manufactured by Asahi Glass Co., Ltd.) was caused to react with 57 partsby mass of coarse MDI (trade name: COSMONATE M-200, manufactured byMitsui Chemical Polyurethane) at 90° C. for 2 hours under heat. Afterthat, butyl cellosolve was added to the resultant so that a solidcontent became 70 mass %. Thus, an isocyanate compound having a massratio of an NCO group incorporated per unit solid content of 5.0 mass %was obtained. After that, 22 parts by mass of MEK oxime was dropped tothe compound under the condition of a reactant temperature of 50° C.Thus, a blocked polyisocyanate was obtained.

[4. Production of Application Liquid for Forming Urethane Layer]

The polyol produced as described above was mixed with the blockedpolyisocyanate so that an NCO/OH group ratio became 1.4. Thus, a rawmaterial for a “polyurethane A” as a resin component was obtained. 100Parts by mass of the resin solid content of the mixture was mixed with acarbon black particle and a ZnO particle (A) shown in Table 2 below, thevolumes of the ZnO particle and the carbon black particle were adjustedto the same value, and the materials were dissolved or dispersed in MEKso that their total solid content became 40 mass %, followed by mixing.The mixed liquid was dispersed and mixed with glass beads each having aparticle diameter of 0.5 mm in a sand mill for 6 hours to produce anapplication liquid for forming the urethane layer.

TABLE 2 Material Part(s) by mass Mixture of polyol and blocked 100polyisocyanate (NCO/OH group ratio = 1.4) Carbon black particle (tradename: MA100, 17 manufactured by Mitsubishi Chemical Corporation, pH =3.5) ZnO particle (trade name: MZ-303S, 50 manufactured by TaycaCorporation, particle diameter: 35 nm)

[5. Formation of Urethane Layer on Silicone Rubber Layer]

The application liquid for forming the urethane layer obtained asdescribed above was loaded into the application liquid tank of a dippingapplication apparatus, and the roller with the silicone layer wasimmersed in the application liquid tank while its uppermost portion washeld with its longitudinal direction defined as a vertical direction.Next, the roller was lifted from the inside of the application liquidtank. Conditions such as the speed at which the roller was lifted wereappropriately set so that the thickness of the urethane layer became adesired value. The roller having the urethane layer applied onto thesilicone layer thus obtained was air-dried at room temperature for 30minutes, and was then thermally treated in a hot air-circulating oven at140° C. for 2 hours and 30 minutes to provide the developing rollerhaving a polyurethane layer on the surface of the silicone layer.

[6. Calculation of Electroconductivity σ and Dielectric Constant ∈]

In the developing roller of this example, the resin j is the urethaneresin (polyurethane A), the electro-semiconductive particle p is the ZnOparticle (A), and the base layer is the silicone rubber. The values ofthe electroconductivities σ_(j) and σ_(p) of the resin j andelectro-semiconductive particle p of the developing roller calculated bythe AC impedance method were 1.2×10⁻¹⁰ S/cm and 1.4×10⁻⁹ S/cm,respectively, and the values of the dielectric constants ∈_(j) and ∈_(p)thereof calculated by the method were 20 and 8, respectively. Thoseelectric parameters satisfy the relational formulae (1) and (2).

It should be noted that both the volume fractions of the urethane resinand the ZnO particle forming the urethane layer were 0.5, and hence thecalculation was performed by setting the values of the parameters a ofthe urethane resin, the ZnO particle, and the base layer to 5.65×10⁻³[m], 5.65×10⁻³ [m], and 3×10⁻³ [m], respectively, and the values of theparameters b thereof to 5.7×10⁻³ [m], 5.7×10⁻³ [m], and 5.65×10⁻³ [m],respectively.

[7. Calculation of σ_(u)/∈_(u)]

In this example, the respective parameters of the resin j and the baselayer b calculated by a complex impedance method were as follows:G_(j)=3.9×10⁻⁶ [S], C_(j)=6.8×10⁻¹⁰ [F], G_(b)=7.7×10⁻⁴ [S],C_(b)=5.9×10⁻¹¹ [F], G_(u)/C_(u)=5.7×10³ [S/F], andσ_(u)/∈_(u)=5.1×10⁻¹⁰ [S/cm].

It should be noted that in the following examples and comparativeexamples as well, the respective impedance characteristic parameterswere similarly calculated.

Example 2

22 Parts by mass of a SiO₂ particle (trade name: MSP-009, manufacturedby Tayca Corporation, particle diameter: 80 nm) was used as theelectro-semiconductive particle p. The volumes of the carbon blackparticle and the SiO₂ particle to be dispersed in the urethane resinwere adjusted so as to be equal to each other. A developing roller 2 wasproduced under the same conditions as those of Example 1 except theforegoing.

Comparative Example 1

The outer periphery of the silicone rubber layer (thickness: 2.74 mm)was coated with a urethane resin layer (thickness: 10 μm) havingdispersed therein a roughening particle and a conductive agent as acoating layer. A developing roller C1 was produced under the sameconditions as those of Example 1 except the foregoing.

Comparative Example 2

A TiO₂ particle (trade name: MT-700B, manufactured by Tayca Corporation,particle diameter: 80 nm) was used as the electro-semiconductiveparticle p to be incorporated into the urethane layer as the secondlayer, and was used in an amount of 33 parts by mass with respect to 100parts by mass of the polyol. The usage of the TiO₂ particle was adjustedso that the volumes of the carbon black particle and the TiO₂ particleto be dispersed in urethane were equal to each other. A developingroller C2 was produced under the same conditions as those of Example 1except the foregoing.

Example 3

A ZnAlO particle (trade name: Pazet CK, manufactured by Hakusuitech Co.,Ltd., particle diameter: 35 nm) was used as the electro-semiconductiveparticle p to be incorporated into the urethane layer as the secondlayer, and was used in an amount of 50 parts by mass with respect to 100parts by mass of the polyol. The usage of the ZnAlO particle wasadjusted so that the volumes of the carbon black particle and the ZnAlOparticle to be dispersed in urethane were equal to each other. Adeveloping roller 3 was produced under the same conditions as those ofExample 1 except the foregoing.

Example 4

The polyol and the blocked polyisocyanate used in Example 1 were mixedso that an NCO/OH group ratio became 0.9. Thus, a raw material for a“polyurethane B” as a resin component was obtained. A developing roller4 was produced in the same manner as in Example 1 except that theurethane raw material was used as a urethane raw material.

Comparative Example 3

A ZnGaO particle (trade name: Pazet GK-40, manufactured by HakusuitechCo., Ltd., particle diameter: 35 nm) was used as theelectro-semiconductive particle p to be incorporated into the urethanelayer as the second layer, and was used in an amount of 50 parts by masswith respect to 100 parts by mass of the polyol. The usage of the ZnGaOparticle was adjusted so that the volumes of the carbon black particleand the ZnGaO particle to be dispersed in urethane were equal to eachother. A developing roller C3 was produced under the same conditions asthose of Example 1 except the foregoing.

Comparative Example 4

An Al₂O₃ particle (trade name: SERATH, manufactured by KINSEI MATEC CO.,LTD., particle diameter: 35 nm) was used as the electro-semiconductiveparticle p to be incorporated into the urethane layer as the secondlayer, and was used in an amount of 50 parts by mass with respect to 100parts by mass of the polyol. The usage of the Al₂O₃ particle wasadjusted so that the volumes of the carbon black particle and the Al₂O₃particle to be dispersed in urethane were equal to each other. Adeveloping roller C4 was produced under the same conditions as those ofExample 1 except the foregoing.

Example 5

A ZnO particle (B) (trade name: LPZINC-2, manufactured by SAKAI CHEMICALINDUSTRY CO., LTD., volume-average particle diameter: 2 μm) was used asthe electro-semiconductive particle p to be incorporated into theurethane layer as the second layer, and was used in an amount of 50parts by mass with respect to 100 parts by mass of the polyol. Adeveloping roller 5 was produced under the same conditions as those ofExample 1 except the foregoing.

Example 6

A urethane resin (product name: UREARNO, model number: KL-593,manufactured by Arakawa Chemical Industries, Ltd.) was used as a rawmaterial for a “polyurethane C” as a resin for forming the urethanelayer as the second layer. In addition, the same particles as those ofExample 1 were used as the electro-semiconductive particle and theelectro-conductive particle to be incorporated into the resin. 100 Partsby mass of the urethane resin was mixed with 1.8 parts by mass of thecarbon black particle and 5.4 parts by mass of the ZnO particle (A), andisopropyl alcohol was added to the mixture so that the total solidcontent became 40 mass %. The mixed liquid was mixed with glass beadseach having a particle diameter of 0.5 mm, and the materials weredispersed and mixed in a sand mill for 6 hours to produce an applicationliquid for forming the urethane layer. The application liquid forforming the urethane layer obtained as described above was applied ontothe silicone layer with a dipping application apparatus by dipping, andwas air-dried at room temperature for 30 minutes. After that, the driedproduct was thermally treated in a hot air-circulating oven at 80° C.for 30 minutes to form the urethane layer. A developing roller 6 wasproduced in the same manner as in Example 1 except the foregoing.

Example 7

The same urethane resin as that of Example 6 was used as the resin forforming the urethane layer as the second layer. In addition, the sameZnO particle (B) as that of Example 5 was used as theelectro-semiconductive particle to be incorporated into the resin, andthe same carbon black particle as that of Example 1 was used as theelectro-conductive particle to be incorporated into the resin. 100 Partsby mass of the urethane resin was mixed with 1.8 parts by mass of thecarbon black particle and 5.4 parts by mass of the ZnO particle (B), andisopropyl alcohol was added to the mixture so that the total solidcontent became 40 mass %. A developing roller 7 was produced in the samemanner as in Example 6 except the foregoing.

Example 8

Materials for electro-conductive elastic layers were as follows: a firstlayer was a silicone rubber layer (thickness: 3.0 mm), a second layerwas a urethane intermediate layer (thickness: 9 μm), and a third layerwas a urethane outermost surface layer (thickness: 1 μm). The samematerials as those of Example 7 were used as the electro-semiconductiveparticle p and the electro-conductive particle c to be incorporated intoeach of the second and third electro-conductive elastic layers. In theurethane intermediate layer, 5.4 parts by mass of the ZnO particle (B)and 1.8 parts by mass of the carbon black particle were incorporatedinto 100 parts by mass of the raw material for the “polyurethane C.” Inaddition, in the urethane outermost surface layer, 10.8 parts by mass ofthe ZnO particle (B) and 1.8 parts by mass of the carbon black particlewere incorporated into 100 parts by mass of the raw material for the“polyurethane C.” A developing roller was produced under the sameconditions as those of Example 7 except the foregoing.

It should be noted that the thickness of each of the urethaneintermediate layer and the urethane outermost surface layer was adjustedto a desired thickness by adjusting the speed at which the roller waslifted at the time of film formation.

[Evaluation Method]

Each of the developing rollers produced in Examples 1 to 8 andComparative Examples 1 to 4 was attached to each of the cartridge ofEmbodiment 1 illustrated in FIG. 2 and the cartridge of Embodiment 2illustrated in FIG. 3, and the following image evaluations wereperformed. The results of the evaluations are shown in Table 3. Itshould be noted that the notation “E−n” means “×10^(−n)”.

[1. Evaluation Method in Embodiment 1]

(Evaluation for Endurance Fogging)

An evaluation for a fogging amount was performed by the followingmethod.

An image forming apparatus was stopped during the printing of a solidwhite image. A toner on a photosensitive drum after development andbefore transfer was transferred onto a transparent tape once, and thetape having adhered thereto the toner was bonded to recording paper orthe like. In addition, a tape to which no toner had adhered wassimultaneously bonded onto the same recording paper. An opticalreflectance R₁ was measured from above the tape bonded to the recordingpaper with the green filter of an optical reflectance-measuring machine(TC-6DS manufactured by Tokyo Denshoku Co., Ltd.), and a value “R₀-R₁”obtained by subtracting the measured value from an optical reflectanceR₀ of the tape to which no toner had adhered was defined as a foggingamount. The measurement was performed at three or more points on thetape, and the average of the three measured values was determined as thefogging amount and ranked as any one of A to E by the followingcriteria.

A: The fogging amount is less than 1.0%.

B: The fogging amount is 1.0% or more and less than 3.0%.

C: The fogging amount is 3.0% or more and less than 5.0%.

D: The fogging amount is 5.0% or more and less than 7.0%.

E: The fogging amount is 7.0% or more.

An evaluation for “endurance fogging” was performed after a printingtest had been performed on 3,000 sheets in a test environment (having atemperature of 30° C. and a relative humidity of 80%), and then theimage forming apparatus had been left to stand for 24 hours. Theprinting test was performed by continuously passing sheets each having ahorizontal line-recorded image having an image ratio of 5%. Here, thehorizontal line having an image ratio of 5% is such an image that thefollowing pattern is repeated: after a one-dot line has been printed, nolines are printed over 19 dots. In addition, the printing test wasperformed according to a normal-speed mode (120 mm/sec), and theevaluation for fogging was performed according to each of thenormal-speed mode (120 mm/sec) and a low-speed mode (60 mm/sec).

[2. Evaluation Method in Embodiment 2]

(2-1. Evaluation for Endurance Fogging in Low-Speed Mode when No Cleaneris Used)

In this evaluation, the evaluation for “endurance fogging” performedaccording to the low-speed mode (60 mm/sec) after the printing test hadbeen performed on the 3,000 sheets in Embodiment 1 was performed. Thisevaluation is in conformity with the evaluation for endurance fogging inEmbodiment 1.

(2-2. Evaluation for Initial Halftone Density when No Cleaner is Used)

This evaluation was performed after an image forming apparatus had beenleft to stand in an evaluation environment (having a temperature of 30°C. and a relative humidity of 80%) for 24 hours to be conformed to theenvironment, and then an image had been printed on 100 sheets. The100-sheet printing test was performed by continuously passing sheetseach having a horizontal line-recorded image having an image ratio of5%. An image evaluation was performed as described below. A halftoneimage was printed on 1 sheet. Next, 10 sheets each having a verticalstripe image having a width of 2 cm were continuously passed, and ahalftone image was printed on an 11th sheet as well by continuous paperpassing. Further, sheets each having the vertical stripe image having awidth of 2 cm were continuously passed, and a halftone image was printedon a 21st sheet as well by continuous paper passing. The printing testand the output of an image to be evaluated were performed according to amonochromatic mode, and the image was output according to thenormal-paper mode (120 mm/sec). The result of the evaluation was rankedas any one of A to C by the following criteria.

A: A density difference between the halftone images on the 1st sheet andthe 21st sheet cannot be visually recognized.

B: A density difference between the halftone images on the 1st sheet andthe 21st sheet can be visually recognized, but a density differencebetween the halftone images on the 1st sheet and the 11th sheet cannotbe visually recognized.

C: A density difference between the halftone images on the 1st sheet andthe 11th sheet can be visually recognized.

In this evaluation, the halftone image means a striped pattern in whicha 1 line is recorded in a main scanning direction and then no recordingis performed over 4 lines, and the image represents a halftone densityas a whole.

TABLE 3 Electro-conductive elastic layer Electro- Electro- Electro-conductivity Dielectric Semiconductive Conductive σ [S/cm] constant εparticle p Resin j particle c σ_(p) σ_(j) ε_(p) ε_(j) σ_(p)/ε_(p)Example 1 ZnO (A) Urethane A Carbon 1.40E−09 1.20E−10 8 20 1.80E−10Comparative — Urethane A Carbon — 1.20E−10 — 20 — Example 1 ComparativeTiO₂ Urethane A Carbon 1.00E−07 1.20E−10 60 20 1.70E−09 Example 2Comparative ZnGaO Urethane A Carbon 5.00E−02 1.20E−10 8 20 6.30E−03Example 3 Comparative Al₂O₃ Urethane A Carbon 2.00E−12 1.20E−10 15 201.30E−13 Example 4 Example 2 SiO₂ Urethane A Carbon 1.60E−10 1.20E−10 1220 1.30E−11 Example 3 ZnAlO Urethane A Carbon 1.00E−07 1.20E−10 8 201.30E−08 Example 4 ZnO (A) Urethane B Carbon 1.40E−09 1.36E−09 8 301.80E−10 Example 5 ZnO (B) Urethane A Carbon 1.50E−10 1.20E−10 6 202.50E−11 Example 6 ZnO (A) Urethane C Carbon 1.40E−09 4.70E−12 8 101.80E−10 Example 7 ZnO (B) Urethane C Carbon 1.50E−10 4.70E−12 6 102.50E−11 Example 8 Surface Urethane C Carbon 1.50E−10 4.70E−12 6 102.50E−11 ZnO (B) Requirement of Embodiment 2 formula Embodiment 1Endurance Halftone Formula Formula Value of left side Normal Low foggingdensity (1) (2) of formula (10) speed speed (low speed) (normal speed)Example 1 Satisfied Satisfied 0.5 B B A A Comparative — — — C D E CExample 1 Comparative Satisfied No 0.5 C D E C Example 2 Comparative NoSatisfied 7.1 C D E C Example 3 Comparative No Satisfied 3.6 C D E CExample 4 Example 2 Satisfied Satisfied 1.6 B B C B Example 3 SatisfiedSatisfied 1.4 B B C A Example 4 Satisfied Satisfied 0.9 B B B A Example5 Satisfied Satisfied 1.3 B B C A Example 6 Satisfied Satisfied 0.7 B BB A Example 7 Satisfied Satisfied 0.2 A A B A Example 8 SatisfiedSatisfied 0.2 A A A A

[Discussion of Results of Evaluations]

[1. Superiority of the Present Invention Over Related Art]

First, the superiority of the present invention over Comparative Example1 as related art is described. In Embodiment 1, the fogging amountincreases in Comparative Example 1 as compared with Example 1. A reasonfor the foregoing is described. A toner having charge is conveyed to adeveloping portion contacting with a photosensitive drum by a developingroller. During the printing of a solid white image, in the contactingportion between the photosensitive drum and the developing roller, anelectric field is applied to the charge on the toner in a directionflowing toward the developing roller. Under a high-humidity environment,the respective resistances of the toner and the developing rollerreduce, and hence a situation where the attenuation of the charge isremarkably liable to occur toward the developing roller is established.In addition, after the endurance running of an image forming apparatus,the chargeability of the toner reduces owing to the deterioration of thetoner. Accordingly, when a reduction in charge of the toner occurs inthe developing portion, the control with the electric field becomesdifficult and the transfer of the toner onto the whitecharacter-printing region of the photosensitive drum accelerates, andhence fogging increases. In Comparative Example 1, the fogging amountincreases by the foregoing reason. In addition, when the resistance ofthe developing roller is increased, the density and gradation change. Incontrast, in the present invention, the attenuation of the charge of thetoner is suppressed by maintaining the average resistance at a propervalue, and hence the fogging amount can be significantly suppressedwithout any image fluctuation.

Mechanisms for the suppression of the attenuation of the toner chargeand the suppression of the fogging in the developing portion of thepresent invention are considered as described below. During the printingof the solid white image, in the developing portion, an electric fieldis applied to the toner having charge on the developing roller in thedirection in which the charge escapes toward the developing roller. A DCvoltage is applied between the developing roller and the photosensitivedrum, but each toner particle on the developing roller passes a regionto which the electric field is applied only when the particle passes thedeveloping portion, and hence the toner itself can be attributed to amodel that temporarily receives an AC electric field (FIG. 15).

Accordingly, like FIG. 15, the capacitance component of the developingroller is considered as a factor of the attenuation of the charge of thetoner. In Example 1, however, the capacitance component of thedeveloping roller can be appropriately reduced and hence the attenuationof the toner charge can be effectively suppressed. In addition, when theapparatus has a mode in which a process speed is slow, a time periodrequired for the toner particle to pass the developing portion lengthensand hence the attenuation of the toner charge is additionallyaccelerated. In Comparative Example 1, the fogging worsens at the timeof the low-speed mode. In contrast, in Example 1, a reduction incapacitance is maintained even in a low-frequency region and hence thefogging amount can be effectively suppressed. Further, Embodiment 2 isan embodiment in which a cleaner container is removed, and the tonerthat cannot be transferred and hence remains on the photosensitive drumpasses a charging portion to be recovered in the developing portion, andthe value of the Vback is set to be as large as 500 V in order that theproperty by which the toner is recovered may be improved. In this case,the electric field to be applied to the toner in the developing portionincreases and hence the attenuation of the toner charge is additionallyaccelerated. Accordingly, in Comparative Example 1, an increase infogging amount occurs to a larger extent than that in Embodiment 1, butin Example 1, the fogging amount can be significantly suppressed. Inaddition, in Comparative Example 1, the amount of the toner remainingafter the transfer due to a fogging toner is large, and hence thecharging roller is contaminated with the toner, a fluctuation in itscharging ability occurs, and a halftone image density fluctuates. Incontrast, in Example 1, the contamination of the charging roller withthe fogging toner can also be suppressed and hence a satisfactoryhalftone image density can be obtained.

[2. Superiority of the Present Invention Over Comparative Technology]

Next, the superiority of the present invention is described by comparingthe present invention with a comparative technology.

First, Comparative Examples 3 and 4 that do not satisfy the formula (1)are described. Comparative Example 4 is an example in which theelectroconductivity σ_(p) of the electro-semiconductive particle pcalculated from the complex impedance method is smaller than theelectroconductivity σ_(j) of the resin. In this case, as illustrated inFIG. 13, the average resistance is increased by theelectro-semiconductive particle p but a substantial electro-conductivepath does not change. On the other hand, Comparative Example 3 is anexample in which the electroconductivity σ_(p) of theelectro-semiconductive particle p calculated from the complex impedancemethod is larger than 5×10⁻² [S/cm]. When the electroconductivity σ_(p)of the electro-semiconductive particle p is large to be comparable tothe numerical value, its conduction may be equivalent to the conductionof carbon and hence the substantial electro-conductive path isequivalent to that of Comparative Example 1 (FIG. 14). Therefore, inComparative Examples 3 and 4 in which the electro-semiconductiveparticle p is substantially free from being involved, the capacitancecomponent of the developing roller cannot be reduced and hence theirresults of the evaluations are equivalent to those of ComparativeExample free of any electro-semiconductive particle. On the other hand,Example 1 as the present invention satisfies the formula (1) and hencethe electro-semiconductive particle p is involved in theelectro-conductive path. Accordingly, a reduction in capacitance of theentirety of the developing roller is realized, and hence in each ofEmbodiments 1 and 2, the fogging amount can be significantly suppressed.

On the other hand, the results of the evaluations for fogging ofComparative Example 2 are equivalent to those of Comparative Example 1as the related art in spite of the fact that Comparative Example 2satisfies the formula (1). A possible reason for the foregoing is thatComparative Example 2 does not satisfy the formula (2). In ComparativeExample 2, the dielectric constant ∈_(p) of the electro-semiconductiveparticle calculated from the complex impedance method is larger than thedielectric constant ∈_(j) of the resin, and hence as described above,the capacitance component of the entirety of the developing roller mayincrease at the time of the formation of the substantialelectro-conductive path. The results of the evaluations are equivalentto those of Comparative Example 1 but slightly worsen as compared withthose of Comparative Example 1. As described above, in the presentinvention, when a relationship between the electroconductivitiescalculated from the complex impedance method satisfies the formula (1),the electro-semiconductive particle p is involved in the substantialelectro-conductive path, and when a relationship between the dielectricconstants calculated from the complex impedance method satisfies theformula (2), the capacitance of the entirety of the developing rollercan be reduced, whereby the fogging can be stably suppressed.

[3. Comparison Between Examples]

Examples 1 to 8 are compared with one another in order that the effectin the present invention may be described. All of Examples 1 to 8 eachsatisfy the relational formulae (1) and (2), and hence the capacitanceof the entirety of the developing roller can be reduced as compared withthat of Comparative Example 1 as the related art and the results of theevaluations for endurance fogging in Embodiment 1 are satisfactory. Onthe other hand, in Embodiment 2, the Vback is large, and hence a statewhere the charge on the toner is liable to attenuate toward thedeveloping roller is established and a slight increase in endurancefogging (low speed) is observed. A reason for the foregoing is describedbelow.

At the time of a low process speed, a time period required for the toneron the developing roller to pass the developing portion contacting withthe photosensitive drum lengthens, and hence a state where the charge ofthe toner is liable to attenuate is established. In other words, theforegoing means that the charging of the capacitance component of thedeveloping roller is liable to proceed. Further, in Embodiment 2, avoltage to be applied to the developing portion is set to a value largerthan that in Embodiment 1 in the direction in which the chargeattenuates toward the developing roller. Accordingly, the capacitancecomponent of the entirety of the developing roller needs to beadditionally suppressed and the capacitance component of the entirety ofthe developing roller needs to be suppressed particularly in alow-frequency band.

The inventors have made extensive investigations, and as a result, havefound that as the value of the left side of the formula (10) reduces, anincrease in capacitance can be suppressed to a larger extent in thelow-frequency band. The value of the ratio [electroconductivityσ]/[dielectric constant ∈] calculated by the impedance method is knownto be proportional to a relaxation frequency. When it is assumed that anelectro-conductive component and a dielectric component form a parallelcircuit, the relaxation frequency describes whether the circuit iselectro-conductive or dielectric; when the relaxation frequency islarge, the circuit is electro-conductive, and when the relaxationfrequency is small, the circuit is dielectric.

The case where a plurality of layers to be separated by the impedancemethod occur like this example can be attributed to a circuit model inwhich the plurality of parallel circuits each having theelectro-conductive component and the dielectric component are arrangedin series (section (a) of FIG. 16).

When the components are synthesized into the electro-semiconductiveparticle p component and the component u except the component out of theplurality of circuits like the section (b) of FIG. 16, a state where thevalue of the left side of the formula (10) is small means that theelectro-semiconductive particle p component and the component u exceptthe component are close to each other in balance between conduction andinduction. In addition, in contrast, a state where the value is largemeans that layers different from each other in balance betweenconduction and induction are laminated. When the layers different fromeach other in balance between conduction and induction are laminated,the speed at which charge is accumulated varies from dielectric layer todielectric layer, and hence frequency dependence occurs. Specifically,in the case of a high-frequency band, the dielectric components of therespective layers are each in a chargeable state (FIG. 17A) becausecharge flows in before the completion of the charging of the dielectriccomponent of each layer with charge. At this time, the entirety of thedeveloping roller can be attributed to a model in which dielectricmaterials are laminated, and an amount corresponding to a thickness d ofthe respective laminated layers is reflected in the capacitance of theentirety of the developing roller (FIG. 17B). On the other hand, in thelow-frequency band, the roller has a sufficient charging time, and hencea layer whose charging is completed and a layer whose charging is notcompleted are liable to occur. In the layer whose charging has beencompleted, the behavior of only a resistance component becomes dominantand the layer is hardly reflected in the capacitance of the entirety ofthe developing roller. Accordingly, only the dielectric component of thelayer whose charging is not completed may be reflected in thecapacitance component of the entirety of the developing roller (FIG.17C). In other words, only an amount corresponding to the thickness ofthe uncharged layer smaller than the thickness d of the layers to beoriginally laminated affects the capacitance component of the entiretyof the developing roller (FIG. 17D). The thickness d of the laminate ofthe respective layers corresponds to the distance d between parallelplate electrodes, and the capacitance C is inversely proportional to thedistance d between the electrodes. The thickness corresponding to theuncharged layer is smaller than the thickness of all layers to belaminated, and hence the capacitance of the entirety of the developingroller increases. As described above, in the case where the layersdifferent from each other in balance between conduction and inductionare laminated, when a frequency band changes, the speed at which chargeis accumulated varies from dielectric layer to dielectric layer, and thethickness of the dielectric layer involved in the entirety of thedeveloping roller changes, whereby the frequency dependence of thecapacitance component of the entirety of the developing roller occurs.On the other hand, in the case where the layers close to each other inbalance between conduction and induction are laminated, even when thefrequency band changes, the dielectric component of each layer isreflected in the capacitance component of the entirety of the developingroller, and hence its dependence on a frequency can be reduced. Further,when the thickness of the entirety of the developing roller does notchange like this example, an excess increase in capacitance component ofthe entirety of the developing roller in the low-frequency band can besuppressed. That is, the foregoing means that as the value of the leftside of the formula (10) reduces, an increase in capacitance can besuppressed to a larger extent in the low-frequency band.

The values of the left side of the formula (10) of Examples 1 to 7 ofthe present invention are described. In Examples 1, 4, 6, and 7, thevalues are as small as 0.5, 0.9, 0.7, and 0.2, respectively, which meansthat an increase in capacitance component of the entirety of thedeveloping roller with a change in frequency is small. As a result, thefogging amount can be stably suppressed even in the low-speed mode ofEmbodiment 2 in which charge is liable to attenuate toward thedeveloping roller. On the other hand, in Example 2 or Example 3, thevalue is somewhat large and hence a slight increase in fogging amountoccurs. A possible reason for the foregoing is that the capacitance ofthe entirety of the developing roller slightly increases in thelow-frequency band. In addition, Example 2 is different from Example 1in that the kind of the electro-semiconductive particle p is changed,and Example 4 is different from Example 1 in that the kind of the resinj is changed. The values of the left side of the formula (10) of boththe examples are larger than the value of Example 1, but the value ofthe left side of the formula (10) of Example 2 is 1.6, which is a valueeven larger that of Example 4, and hence an increase in fogging in thelow-speed mode occurs. In addition, in Example 3 as well, the value ofthe left side of the formula (10) is large and hence a slight increasein fogging occurs, but no slight increase in halftone image failure atthe normal speed is observed. In addition, as illustrated in FIG. 11,the frequency dependence of the capacitance of the developing roller isfound to reduce in association with the value of the left side of theformula (10). It should be noted that the notation “E+n” means “×10^(n)”and the notation “E−n” means “×10^(−n)”.

In Example 4, the value of the left side of the formula (10) is 0.9,which is a relatively low value, and hence neither an increase infogging in the low-speed mode nor a halftone image failure is observed.It is because of the following reason that the value of the left side ofthe formula (10) slightly increases as compared with that of Example 1:in Example 4, the NCO/OH group ratio of the urethane resin is smallerthan that of Example 1, and hence the electroconductivity σ_(j) of theresin is large, and a difference in balance between conduction andinduction between the layers constituting the developing rollerenlarges.

Example 5 is different from Example 1 in that the kind of the ZnOparticle as the electro-semiconductive particle p is changed. A mainfactor of an increase in value of the left side of the formula (10) isthat the electroconductivity σ_(p) of the ZnO particle (B) is smallerthan that of the ZnO particle (A) used in Example 1 and hence the valueof the ratio (σ_(p)/∈_(p)) deviates from that of the ratio(σ_(u)/∈_(u)). A factor of the fact that even similar ZnO particlesdiffer from each other in electroconductivity like the ZnO particle (A)and the ZnO particle (B) is considered as described below. A ZnOparticle has a hexagonal crystal structure, and in most of the ZnOparticles commercially available in powder shapes, defective portions ofthe crystal structure such as the missing of an O particle are present.The electroconductivity of a particle tends to reduce as the number ofsuch defective portions reduces and hence its crystallinity becomeshigher. The ZnO particle used in Example 5 may have a lowerelectroconductivity because its crystallinity is higher than that of theparticle used in Example 1.

Example 6 is different from Example 1 in that the “polyurethane C” isused as a raw material for the resin j. A main factor of an increase invalue of the left side of the formula (10) is that theelectroconductivity σ_(j) of the polyurethane C is smaller than that ofthe polyurethane A used in Example 1 and hence the value of the ratio(σ_(u)/∈_(u)) deviates from that of the ratio (σ_(p)/∈_(p)). A factor ofthe fact that the electroconductivity of the polyurethane C is lowerthan that of the polyurethane A is considered as described below. Theresults of the measurement of the water absorption amounts of thedeveloping roller of Example 1 (using the polyurethane A) and thedeveloping roller of Example 6 (using the polyurethane C) are shown inFIG. 18. It is found from FIG. 18 that the water absorption ratio of thedeveloping roller of Example 6 is smaller than that of Example 1. Thatis, it is found that the hydrophobicity of the polyurethane C is higherthan that of the polyurethane A. It is assumed from the foregoing thatthe polyurethane C suppresses the water absorption of the resin in thehigh-temperature and high-humidity environment (having a temperature of30° C. and a relative humidity of 80%) where this evaluation has beenperformed, and hence suppresses increases in electroconductivity anddielectric constant due to water. Accordingly, the resin may have thefollowing characteristics even under the environment: a lowelectroconductivity and a low dielectric constant. A method of measuringa water absorption amount is described below.

[Measurement of Water Absorption Amount]

The measurement of a water absorption amount was performed with acalorimeter (trade name: Model Q5000, manufactured by TA Instruments). Adeveloping roller was cut in its longitudinal direction so as to have awidth of 1 mm (see FIG. 20A). After that, a section was cut out of thecut surface so as to measure 8 mm in the widthwise direction of theroller and 1.6 mm from its surface in its thickness direction, and wasdefined as a sample piece to be used in the measurement of the waterabsorption amount (see FIG. 20B). Measurement conditions are shown inFIG. 19. As an indicator for evaluating a water-absorbing property froma temperature of 15° C. and a relative humidity of 10% to a temperatureof 30° C. and a relative humidity of 80%, a water absorption ratio wasdefined as described below by using a mass M0 of the sample at atemperature of 15° C. and a relative humidity of 10%, and a mass M1 ofthe sample at a temperature of 30° C. and a relative humidity of 80%.Water absorption ratio [%]=(M1−M0)/M0×100

The inventors of the present invention have made extensiveinvestigations, and as a result, have confirmed that when a urethaneresin whose water absorption ratio measured by the method is 0.045% orless is used, the resin shows the following characteristics in ahigh-temperature and high-humidity environment (having a temperature of30° C. and a relative humidity of 80%): a low electroconductivity and alow dielectric constant.

Example 7 is different from Example 1 in that the kind of the ZnOparticle as the electro-semiconductive particle p and the kind of thepolyurethane as the resin j are changed. The result of the evaluationfor endurance fogging in Embodiment 1 is significantly satisfactorybecause the value of the left side of the formula (10) is 0.2, which isan extremely low value. Examples 5, 6, and 7 are developing rollers inwhich the combination of the electro-semiconductive particle p and theresin j is changed, and the value of the left side of the formula (10)is lowest in Example 7. A factor of the foregoing is that the values ofthe electroconductivities and dielectric constants of theelectro-semiconductive particle p and the resin j used in Example 7 werea preferred combination for bringing the respective layers constitutingthe developing roller close to each other in balance between conductionand induction. Specifically, in Example 7, the value of the ratioσ_(p)/∈_(p) was 2.5×10⁻¹¹ and the value of the ratio σ_(u)/∈_(u) was3.9×10⁻¹¹, which were values extremely close to each other, and hencethe value of the left side of the formula (10) became 0.2, which was alow value.

As can be seen from the foregoing, in the present invention, it ispreferred that a low-capacitance material involved in anelectro-conductive path be selected, and a developing roller be formedof materials close to each other in balance between a dielectriccomponent and an electro-conductive component in accordance with therespective materials. Specifically, the relationship represented by theformula (10) is preferably satisfied. In addition, in order thatfrequency characteristics may be improved, the value of the left side ofthe formula (10) is more preferably 1.0 or less.

Further, in Example 8, a satisfactory image can be obtained. In Example8, the zinc oxide particle is exposed to the surface of the developingroller and hence charge-providing performance for a toner improves. As aresult, in addition to the suppressing effect of the present inventionon the attenuation of the charge of the toner, a charge-providing effecton the toner is obtained and hence the suppression of the fogging amountcan be effectively performed.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-134823, filed Jun. 30, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A developer carrying member, comprising: anelectro-conductive mandrel; and an electro-conductive elastic layer,wherein: the elastic layer contains a resin j, an electro-semiconductiveparticle p, and an electro-conductive particle c; and when anelectroconductivity of the resin j is defined as σ_(j), a dielectricconstant of the resin j is defined as ∈_(j), an electroconductivity ofthe electro-semiconductive particle p is defined as σ_(p), and adielectric constant of the electro-semiconductive particle p is definedas ∈_(p), σ_(j), ∈_(j), σ_(p), and ∈_(p) satisfy relationshipsrepresented by the following formulae (1) and (2), σ_(j), ∈_(j), σ_(p)and ∈_(p) being calculated by an AC impedance methodσ_(j)<σ_(p)<0.05 S/cm  (1)∈_(p)<∈_(j)  (2).
 2. A developer carrying member according to claim 1,wherein in the electro-conductive elastic layer, when anelectroconductivity of the electro-semiconductive particle p componentis defined as σ_(p), a dielectric constant of the electro-semiconductiveparticle p component is defined as ∈_(p), an electroconductivity of acomponent u except the electro-semiconductive particle p is defined asσ_(u), and a dielectric constant of a component u except theelectro-semiconductive particle p is defined as ∈_(u), σ_(p), ∈_(p),σ_(u), and ∈_(u) have a relationship represented by the followingformula (10),|log₁₀[(σ_(p)/∈_(p))/(σ_(u)/∈_(u))]|<1.5  (10) Wherein σ_(p), ∈_(p),σ_(u), and ∈_(u) being calculated through separation of constituentcomponents of the layer into the electro-semiconductive particle pcomponent and the component u except the electro-semiconductive particlep by the AC impedance method.
 3. A developer carrying member accordingto claim 1, wherein the resin j, the electro-semiconductive particle p,and the electro-conductive particle c incorporated into theelectro-conductive elastic layer are a urethane resin, a zinc oxideparticle, and a carbon particle, respectively.
 4. A developer carryingmember according to claim 3, wherein a volume occupancy of the zincoxide particle in the electro-conductive elastic layer increases towarda vicinity of a surface of the electro-conductive elastic layer.
 5. Animage forming apparatus, comprising: an image-bearing member configuredto bear an electrostatic latent image; and a developer carrying memberconfigured to carry a developer and to contact with the image-bearingmember to develop the electrostatic latent image with the developer,wherein: the developer carrying member comprise: an electro-conductivemandrel; and an electro-conductive elastic layer, wherein: the elasticlayer contains a resin j, an electro-semiconductive particle p, and anelectro-conductive particle c; and when an electroconductivity of theresin j is defined as σ_(j), a dielectric constant of the resin j isdefined as ∈_(j), an electroconductivity of the electro-semiconductiveparticle p is defined as σ_(p), and a dielectric constant of theelectro-semiconductive particle p is defined as ∈_(p), σ_(j), ∈_(j),σ_(p), and ∈_(p) satisfy relationships represented by the followingformulae (1) and (2), σ_(j), ∈_(j), σ_(p) and ∈_(p) being calculated byan AC impedance methodσ_(j)<σ_(p)<0.05 S/cm  (1)∈_(p)<∈_(j)  (2).